1. Field of the Invention
This invention generally relates to a sensing device for sensing the environmental status change, and, in particular, to a sensor for sensing the presence of a particular gas or vapor, such as alcohol, or a level of temperature or humidity. More particular, the present invention relates to a sensing device including a sensing element, the change in electrical resistance of which is utilized for sensing of a desired parameter.
2. Description of the Prior Art
A gas sensor for sensing the presence of a particular gas in the environment over a predetermined amount is widely used. A typical prior art gas sensor includes a sensing element comprised of a metal oxide semiconductor and a heat coil, which also serves as an electrode, embedded in the sensing element. Under the condition that the sensing element is heated to a predetermined temperature by the heater coil, when the sensing element absorbs at its surface a particular gas in the atmosphere, the level of the electrical resistance of the sensing element drops significantly. Thus, this may be utilized to indicate the presence of a particular gas in the atmosphere beyond a predetermined level. In the prior art device, the driving current applied to the heater coil is in the form of a pulse, thereby decreasing power consumption to allow the usage of a battery as a power source.
In the prior art device, the electrical resistance of the sensing element is detected during the time period in which the sensing element is heated. In this case, however, the driving current supplied to the heater coil partly leaks through the sensing element, and such a leak current causes an apparent reduction of an output detection signal from the sensing element. For example, if the resistance of the sensing element under normal condition exhibits approximately 1 M-ohms and it decreases to 500 K-ohms when sensing a particular gas, the application of a driving current of 2 micro-amperes to the sensing element would produce an output voltage of 2 Volts under normal condition and of 1 Volt when sensing a particular gas. However, with the presence of the above-mentioned leak current, since it is equivalent to the situation, in which a resistor having the resistance of 500 K-ohms is connected in parallel with the sensing element, so that the total resistance is approximately 333 K-ohms under normal condition and 250 K-ohms during gas detection. As a result, for the same driving current of 2 micro-amperes, the output voltage will change only from 0.67 Volts to 0.5 Volts.